blit: Add a case for 8-bit blits that sdl12-compat exposed.

sdl12-compat can get into a state where a color-keyed surface is
marked for blending, but wants to blend with full alpha (which
is the same as _not_ blending), so rather than fail to find a
blitter in that case, it just selects the colorkey blitter.

Reference https://github.com/libsdl-org/sdl12-compat/issues/233

(cherry picked from commit 0eea92c8fc)

src/video/SDL_blit_1.c

@@ -531,6 +531,9 @@ SDL_CalculateBlit1(SDL_Surface *surface)
     case SDL_COPY_COLORKEY:
         return one_blitkey[which];
 
+    case SDL_COPY_COLORKEY | SDL_COPY_BLEND:  /* this is not super-robust but handles a specific case we found sdl12-compat. */
+        return (surface->map->info.a == 255) ? one_blitkey[which] : (SDL_BlitFunc)NULL;
+
     case SDL_COPY_MODULATE_ALPHA | SDL_COPY_BLEND:
         /* Supporting 8bpp->8bpp alpha is doable but requires lots of
            tables which consume space and takes time to precompute,